Cusp filter

ABSTRACT

A cusp filter for altering a multi-species plasma to separate ions of different masses (M 1  and M 2 ) includes first and second axi-symmetric magnetic fields which are coaxial, have the same magnitude (B), and are oriented back-to-back to establish a null cusp. The null cusp is thus oriented perpendicular to the axis between the magnetic fields. An injector is provided for directing the plasma ions along the axis toward the null cusp to divert the ions (M 1 ) away from the axis and prevent them from crossing the null cusp, while allowing the ions (M 2 ) to cross the null cusp and proceed along the axis through the filter. In one embodiment, a cut-off mass, M c , is determined such that M 1 &lt;M c &lt;M 2  with M c =e 2 B 2 r 2 /2W where “e” is the ion charge, “r” is the radial distance of the ion from the axis, and W is its kinetic energy. In another embodiment, ions of selected mass are heated by cyclotron resonance to raise their energies above that of other ions in order to assure their passage through the null cusp. The selected ions then pass through the null cusp for separation from the other ions.

FIELD OF THE INVENTION

The present invention pertains generally to devices which are useful forseparating particles of a multi-species plasma according to theirrespective masses. More particularly, the present invention pertains toplasma mass filters which establish magnetic field configurations thatdirect charged particles along predetermined paths according to the massof the specific particle. The present invention is particularly, but notexclusively, useful as a filter for a multi-species plasma thatestablishes a magnetic barrier which prevents selected particles fromproceeding along a predetermined axial path through the filter.

BACKGROUND OF THE INVENTION

It can be mathematically shown that the constants of motion for acharged particle (e.g. an ion) in an axially symmetric magnetic fieldare its angular momentum, P, and its kinetic energy, W. Mathematically,using a cylindrical coordinate system [r, θ, z], these constants ofmotion can be expressed as:

P=Mrv _(θ) +eψ

W=[M/2][v _(r) ² +v _(θ) ² +v _(z) ²]

Where

“M” is the mass of the particle;

“r” is the radial distance of the particle from the axis;

“e” is the charge on a particle (ion);

“ψ” is the flux function of the magnetic field; and

“v” is velocity of the particle (v_(r), v_(θ), and v_(z) are componentsof “v”).

Because the above expressions are general statements of the constants ofmotion, they are applicable to various situations and conditions.Specifically, for a configuration wherein two, otherwise substantiallyidentical, axially symmetric magnetic fields are positioned co-axially,in an opposed back-to-back relationship, the above equations areapplicable. For such a configuration, a null cusp is created in a planeperpendicular to the axis wherein the flux function, ψ, is equal tozero. Stated differently, the flux function on opposite sides of thenull will have opposite signs in the axial (z) direction. As aconsequence of this condition, a charged particle is able to cross thecusp only if it has the necessary momentum and energy to do so.

Because both the momentum and the energy of a particle are functions ofthe mass of the particle, and due to the fact there will be aconservation of the particle's momentum and energy in a system, anexpression can be mathematically derived which will relate the mass ofthe particle to its ability to cross through a null cusp. Here, ofcourse, we are considering the null cusp as described above.Specifically, in this context, for a given energy, W, and for a givenmagnetic field magnitude, B, a cut-off mass, M_(c), can be identifiedsuch that particles with a mass M₂ greater than M_(c) (M₂>M_(c)) willcross the null cusp, while particles with a mass M₁ less than M_(c)(M₁<M_(c)) will not cross the null cusp. The expression for this M_(c)is:

M _(c) =e ² B ² r ²/2W.

In another aspect of particle physics, it is well known that a chargedparticle in a magnetic field will have a cyclotron frequency, f, whichcan be mathematically expressed as: f=Be/2πM. Further, it is known thatall charged particles are subject to cyclotron resonance heating whereina charged particle (electrons or ions) will selectively absorb energy byresonance coupling. Importantly, this resonance coupling is a functionof the mass of the particle. Therefore, all ions of a predetermined massin a multi-species plasma can be selectively heated by resonancecoupling, while ions of other masses are not so heated.

In the environment of the opposed axi-symmetric magnetic fieldsdescribed above, it is to be appreciated that a charged particle (ion)can have either of two types of obits. In a so-called type-1 orbit, theprojection of the orbit onto a plane perpendicular to the magnetic fielddoes not encircle the origin. In this case (type-1 orbit) the angularmomentum, P, and the magnetic flux function, ψ, have the same sign (i.e.Pψ>0). Also, Mrv_(θ)is of opposite sign but is less than the fluxfunction ψ(i.e. |P|<|ψ|). On the other hand, in a type-2 orbit theprojection of the orbit onto a plane perpendicular to the magnetic fieldencircles the origin. In this case (type-2 orbit) the angular momentum,P, and the magnetic flux function, ψ, have opposite signs (i.e. Pψ<0).In this case, Mrv_(θ)is greater in magnitude than the flux function ψand is of opposite sign (i.e. |P|<|ψ|). It can be mathematically shownthat the switch between a type-1 orbit and a type-2 orbit involves alarge change in the angular momentum P. A consequence of this is thatthe orbit of a particle must change from type-1 to type-2, or viceversa, as a particle crosses through a null cusp.

It happens that the concepts discussed above regarding axi-symmetricmagnetic fields, cyclotron resonance heating, and different type orbits,are not mutually exclusive. Specifically, for purposes of separating thecharged particles of a multi-species plasma from each other according totheir respective masses, the concepts just discussed can be usedinterrelatedly. In one application, the energies (W) of chargedparticles in a multi-species plasma can be used to establish a cut-offmass, M_(c), where M₁<M_(c)<M₂ with M_(c)=e²B²r²/2W, so that lower massions, M₁, will not cross the cusp, but the higher ions, M₂, will. Inanother application, selected particles of mass M_(s), in amulti-species plasma, can have their energy and momentum raised bycyclotron resonance heating so that only particles having the selectedmass, M_(s), will cross the cusp. In this second application, theexpression for the cut-off mass is normalized such that withM_(c)/M_(s)=1=e²B²r_(s) ²/2W_(s)M_(s).

In light of the above, it is an object of the present invention toprovide a cusp filter which will selectively heat ions of a particularmass in a multi-species plasma so that the selected particles can beseparated from other particles in the plasma. Another object of thepresent invention is to provide a cusp filter wherein particles selectedfor separation from other particles have their energy and momentumelevated above other particles in a multi-species plasma by cyclotronresonance heating. Yet another object of the present invention is toprovide a cusp filter which establishes a magnetic field configurationwherein a cut-off mass, M_(c), can be determined so that particleshaving masses greater than M_(c) will be influenced differently thanparticles having masses less than M_(c) to thereby separate theparticles of different mass from each other. Still another object of thepresent invention is to provide a cusp filter which is relatively easyto manufacture, simple to use, and comparatively cost effective.

SUMMARY OF THE PREFERRED EMBODIMENTS

A cusp filter in accordance with the present invention includescomponents for generating a magnetic null cusp that is located betweenopposed, axi-symmetric, back-to-back magnetic fields. Both of theback-to back magnetic fields in this case have equal magnitudes that aresubstantially equal to “B.” Their respective magnetic field lines,however, are oriented in opposite directions along their mutual axis.With these orientations, the two magnetic fields establish a magneticnull cusp between them, in a plane that is oriented substantiallyperpendicular to the axis. As contemplated by the present invention, theopposed back-to-back magnetic fields are each generated in the chamberof a container, by a respective plurality of magnetic coils which aremounted on the container.

The cusp filter of the present invention also includes an injector. Inaddition to generating a multi-species plasma, the purpose of thisinjector is to direct both relatively low mass ions (M₁) and relativelyhigh mass ions (M₂) in the multi-species plasma along the axis in thechamber toward the null cusp. As contemplated for the present invention,the separation of ions at the null cusp according to their respectivemasses can be initiated in either of two ways.

For one embodiment of the present invention, differences in either theenergy or the momenta of ions in the multi-species are exploited toseparate ions of mass (M₁) from ions of mass (M₂). More specifically,due to the relatively low energy, or momentum, of the low mass ions (M₁)they are prevented from crossing the null cusp. Instead, they arediverted away from the axis by the null cusp for subsequent collection.On the other hand, the relatively high energy, or momentum, of the highmass ions (M₂) will allow these ions to cross the null cusp and proceedalong the axis through the filter chamber for subsequent collection. Forthis particular embodiment of the present invention, the magnitude, B,of the magnetic fields can be selected to identify a cut-off mass,M_(c), such that M₁<M_(c)<M₂. The expression M_(c)=e²B²r²/2W can then beapplied where “e” is the ion charge, “r” is the radial distance of anion (charged particle) from the axis in the first magnetic. field, and Wis the kinetic energy of the ion. In accordance with the expression forM_(c), it will appreciated that the cusp filter can achieve its intendedresult if either the energy of the ions (M₁) is substantially equal tothe energy of the ions (M₂), or the ions (M₁) and (M₂) are directedtoward the null cusp at a substantially common axial velocity.

In an alternate embodiment of the present invention, ions of a selectedmass, M_(s), can be specifically targeted for separation from other ionsin a multi-species plasma. Importantly, this can be accomplishedregardless whether the selected ions are of comparably higher or lowermass. To do this, the ions of selected mass, M_(s), are heated bycyclotron resonance. The energy of the resonance heated ions is therebyraised substantially above the energies of the other, non-selected ionsin the multi-species plasma. As contemplated by the present invention,cyclotron resonance is accomplished using a cyclotron harmonicsaccelerator, such as a quadrant antenna. For this purpose, the quadrantantenna is operated at twice the resonant cyclotron frequency of theselected ions (2f). Consequently, due to the higher energies of theselected ions, when the multi-species plasma is directed toward the nullcusp, the resonance heated ions will cross the cusp and continue theirtransit through the filter. The other ions, however, having lowerenergy, will be diverted from the filter by the null cusp and they willthereby be separated from the selected ions.

For either embodiment of the present invention, the cusp filter of thepresent invention will include a vacuum pump which is connected to thecontainer. Specifically, the vacuum pump is used to maintain themulti-species plasma below a collisional density in the chamber. Forpurposes of the present invention, this collisional density is definedas a density wherein an ion can cross the null cusp before suffering acollision with another ion. Hence, the collisional density is achievedin a condition wherein the ratio of the collision frequency of an ion toits cyclotron frequency is less than the ratio of the distance of theion from the axis, r, to the axial distance between the ion and the nullcusp.

Several additional aspects of the cusp filter will apply regardless ofits particular embodiment. For one, the cusp filter will include aradial collector that is mounted on the container and orientedsubstantially in the plane of the null cusp. As so positioned the radialcollector is used for collecting ions as they are diverted away from theaxis. Additionally the cusp filter can include an axial collector thatis positioned substantially on the axis for collecting the ions as theyproceed along the axis through the filter. In another aspect, the cuspfilter can include a plurality of electrodes positioned on the containerto bias the magnetic field immediately downstream from the injector toproduce a radial electric field for uniformly increasing the energies ofthe ions in the multi-species plasma to reduce the sensitivity of M_(c)to r. Finally, it is also possible for the cusp filter to incorporate anaxi-symmetric third magnetic field which is coaxial with the first pairof back-to-back magnetic fields. If this is done, the third magneticfield will have a magnitude substantially equal to B, and it will havemagnetic field lines that are oriented in opposition to the magneticfield lines of the middle magnetic field. With this configuration, asecond null cusp will be established to divert ions away from the axis,in a manner as described above, to enhance the separation of ions in themulti-species plasma.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention, as well as the invention itself,both as to its structure and its operation, will be best understood fromthe accompanying drawings, taken in conjunction with the accompanyingdescription, in which similar reference characters refer to similarparts, and in which:

FIG. 1 is a perspective view of a cusp filter in accordance with thepresent invention;

FIG. 2 is a cross sectional view of the cusp filter with representativemagnetic field lines as seen along the line 2—2 in FIG. 1;

FIG. 3 is a view of the cusp filter as seen in FIG. 2 withrepresentative ion paths superposed thereon;

FIG. 4 is a schematic drawing of the ion paths illustrated in FIG. 3 asthey would be seen along the line 4—4 in FIG. 3; and

FIG. 5 is a schematic drawing of a quadrant antenna for a cyclotronharmonics accelerator as seen along the line 5—5 in FIG. 1 wherein atype-1 ion orbit and a type-2 ion orbit are shown in their respectiverelationship with the central axis of the cusp filter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1, a magnetic cusp filter in accordance withthe present invention is shown and is generally designated 10. As shown,the cusp filter 10 includes an elongated cylindrical container 12 whichsurrounds a plasma chamber 14 (see FIG. 2). At one end of the container12, there is an injector 16 for generating a multi-species plasma whichis to be introduced into the chamber 14. For purposes of the presentinvention, it is to be appreciated that the multi-species plasma willinclude at least two different type ions. One type ion has a relativelylow mass (M₁), while the other type ion has a relatively high mass (M₂).

FIG. 1 also shows that the cusp filter 10 of the present invention caninclude a cyclotron harmonics accelerator 18. For purposes of thepresent invention, the cyclotron harmonics accelerator 18 is preferablya quadrant antenna, and is of a type well known in the pertinent art.Importantly, the cyclotron harmonics accelerator 18 should be capable ofheating ions of a predetermined mass, after the ions have beenintroduced by the injector 16 into the chamber 14. Specifically, thisheating is done by establishing resonance with the selected ions at afrequency which is twice the resonant cyclotron frequency, f, of theselected ions.

Still referring to FIG. 1, it is seen that the cusp filter 10 includes afirst magnetic assembly which comprises a plurality of coaxial magneticcoils, of which the magnetic coils 20 a, 20 b and 20 c are exemplary.More specifically, the magnetic coils 20 are mounted on the container 12to generate a magnetic field 22 inside the chamber 14 of container 12(see FIG. 2). Importantly, the magnetic field 22 has a magnitude, B, andthe magnetic field lines of the magnetic field 22 are generally orientedalong the central axis 28 of the container 12, in the direction of thearrow 24 (see both FIG. 1 and FIG. 2). Also, it can be seen in both FIG.1 and FIG. 2 that a plurality of coaxial electrodes 26, of which theelectrodes 26 a, 26 b and 26 c are exemplary, can be mounted on thecontainer 12. As intended for the cusp filter 10 of the presentinvention, the electrodes 26 a-c are used to bias the magnetic fieldlines 22 during operation of the cusp filter 10, if desired.

Still referring to FIG. 1, it will be seen that the cusp filter 10includes a radial collector 30. Further, it is seen in FIG. 1 thatanother plurality of magnetic coils 32 are mounted on the container 12on the side of the radial collector 30 that is opposite from theplurality of magnetic coils 20. Like the coils 20 a-c, the coils 32 a,32 b and 32 c are only exemplary. As perhaps best appreciated by crossreferencing FIG. 1 with FIG. 2, the magnetic coils 32 a, 32 b and 32 care used to generate a magnetic field 34 whose magnetic field lines areoriented in a direction 36 that is axially opposed to the direction 24of the magnetic field 22. The result of these opposed orientations forthe respective magnetic fields 22 and 34 is the creation of a magneticnull cusp 38. Specifically, the null cusp 38 so-created will lie in aplane that is substantially perpendicular to the axis 28. Also, themagnetic flux function, ψ, in the null cusp 38 will be equal to zero(ψ=0). Accordingly, the magnetic field 22 will create a region on oneside of the null cusp 38 where the magnetic flux function is positive(+ψ), while the magnetic field 34 on the other side of the null cusp 38will create a region wherein the magnetic flux function is negative(−ψ).

Although the magnetic fields 22 and 34, alone, are capable ofaccomplishing the objects of the present invention, in an alternateembodiment of the present invention, additional magnetic fields may beused to establish additional null cusps. As shown in FIG. 1, anadditional radial collector 40 is provided between the magnetic coils 32a-c and an additional plurality of magnetic coils 42 a-c. Morespecifically, the magnetic coils 42 a-c are mounted on the container 12substantially as shown, to generate a magnetic field in the chamber 14which has magnetic field lines that are generally oriented along theaxis 28 in the direction of the arrow 44. Consequently, a null cuspsimilar to the null cusp 38 at radial collector 30 will be establishedin the area of the radial collector 40. Regardless of the number of nullcusps that are established for the filter 10, a terminal collector 46can be positioned at the end of the chamber 14 opposite the injector 16to collect ions which are not diverted into a radial collector 30, 40 bya null cusp 38 during the transit of the ions through the chamber 14.Additionally, an axial collector 50 can be employed as shown in FIG. 2.Specifically, the axial collector 50 is positioned on the axis 28 tocollect ions traveling through the filter 10 along the axis 28 which arenot effectively influenced by the combined effects of the magnetic field22 and the null cusp 38. A similar axial collector can likewise be usedin combination with the radial collector 40, for an embodiment using acollector 40.

FIG. 1 also indicates that a vacuum pump 48 is connected in fluidcommunication with the chamber 14. Importantly, the vacuum pump 48 isoperated in concert with the injector 16 to maintain a collisionaldensity inside the chamber 14. For purposes of the present invention,this collisional density is defined as a density wherein an ion cancross a null cusp (e.g. null cusp 38) before suffering a collision withanother ion. Hence, the collisional density satisfies a conditionwherein the ratio of a collision frequency of an ion, to its cyclotronfrequency is less than the ratio of the radial distance r of an ion fromthe axis 28 to the axial distance of the ion to the null cusp 38.

Operation

In the operation of the filter 10, the injector 16 is activated togenerate a multi-species plasma which includes a species of ions havinga first mass (M₁), and a species of ions having a second mass (M₂). Forpurposes of disclosure, the mass (M₂) is considered to be greater thanthe mass (M₁). The object then, is for the filter 10 to separate ions ofmass (M₁) from ions of mass (M₂).

In one aspect of the present invention, the ions of different mass canbe separated from each other by the filter 10 because of differences intheir respective momenta. For example, referring to FIG. 3, initiallyconsider an ion of mass (M₁) located at the point 52 in chamber 14.Also, consider an ion of mass (M₂) located at the point 54. Further,consider that both ions (M₁ and M₂) satisfy the mathematics set forthabove wherein the magnitude of the magnetic field 22 is B. As disclosedabove, a cut-off mass, M_(c), can be determined in the magnetic field 22such that M₁<M_(c)<M₂ with M_(c)=e²B²r²/2W, where “e” is the ion charge,“r”=0 is the radial distance of the ion (charged particle) from the axis28 in the first magnetic field 22, and W is the kinetic energy of theion. Further, the electrodes 26 a-c can be activated to bias magneticfield 22. Specifically, this biasing can be done to produce a radialelectric field which will uniformly increase the energies of the ions(M₁) and the ions (M₂). In turn, these increased energies reduce thesensitivity of M_(c) to r and thereby enhance the effectiveness of theion separation.

Under the conditions just described, it happens that the ion of mass(M₁) does not have sufficient momentum to cross the null cusp 38.Consequently, it will follow an orbital path 56 which diverts the ion ofmass (M₁) from the null cusp 38 and into the radial collector 30. On theother hand, the ion of mass (M₂) will have sufficient momentum to carryit across the null cusp 38 along the orbital path 58. Thus, the ions ofmass (M₁) can be separated from the ions of mass (M₂).

As mentioned above, depending on its energy and momentum, it is possiblefor an ion to have either of two types of orbits inside the chamber 14.In the case just mentioned, both the ion of mass (M₁) and the ion ofmass (M₂) are in type-1 orbits before they reach the null cusp 38. Asshown, the orbital path 56 for the ion of mass (M₁) does not cross thenull cusp 38 and, thus, the ion of mass (M₁) remains in a type-1 orbit.Recall, in a type-1 orbit, the projection of the orbital paths 56, 58onto a plane perpendicular to the magnetic field 22 will not encirclethe axis 28. In this case (type-1 orbit) the angular momentum, P, andthe magnetic flux function, ψ, have the same sign (i.e. Pψ>0). On theother hand, as the ion of mass (M₂) crosses the null cusp 38, itsorbital path 58 changes from a type-1 orbital path 58 into a type-2orbital path 58′. As mentioned above, in a type-2 orbit the projectionof the orbital path 58′ onto a plane perpendicular to the magnetic field34 will encircle the axis 28. In this case (type-2 orbit) the angularmomentum, P, and the magnetic flux function, ψ, have the opposite sign(i.e. Pψ<0). Importantly, as shown with the mathematics as discussedearlier, the orbit of a particle must change from type-1 to type-2, orvice versa, as the particle crosses through the null cusp 38. Anillustration of the above is provided by cross referencing FIG. 3 andFIG. 4.

In another aspect of the present invention, it is possible toselectively raise the energy and momentum of ions (charged particles)having a predetermined mass, by resonance heating. The purpose here isto sufficiently raise the energy and momentum of the selected ions to apoint which will allow them, but not other ions in the multi-speciesplasma, to cross the null cusp 38. The resonance heating that isnecessary to accomplish this can be done using the cyclotron harmonicsaccelerator 18. For example, consider the case wherein the ions (M₁)have a first resonant frequency (f₁) and the ions (M₂) have a secondresonant frequency (f₂). If it is desired that the ions of mass (M₁) becollected by the radial collector 30, and that the ions of mass (M₂)pass through the filter 10 for collection at the terminal collector 46,the cyclotron harmonics accelerator 18 can be operated to selectivelyresonate with the ions (M₂). It happens that this selective heating willsubstantially raise the energy of the ions (M₂) above the energy of theions (M₁).

Referring now to FIG. 5, a schematic of a cyclotron harmonicsaccelerator 18 which can be used in the filter 10 of the presentinvention is shown. Preferably, the accelerator 18 is a quadrant antennaof a type well known in the art, and it is operable at twice theresonant cyclotron frequency of the ions which are selected forresonance heating. For example, if the ion of mass (M₂) are to beselectively heated, the operational frequency of the cyclotron harmonicsaccelerator 18 will be 2f₂. With this in mind, consider an ion of mass(M₁) at the point 52 in the chamber 14, and an ion of mass (M₂) at thepoint 54 (see both FIG. 3 and FIG. 5). Even though they might nototherwise be able to cross the null cusp 38, due to the resonanceheating provided by the cyclotron harmonics accelerator 18, the energyand momenta of the ions of mass (M₂) will be raised sufficiently toallow them to cross the null cusp 38. This, however, does not happen tothe ions of mass (M₁) because they will not effectively respond to thefrequency of 2f₂. Recall, the ions (M₁) have a different resonantfrequency (f₁). A consequence of this is that, before crossing the nullcusp 38, the ions of mass (M₂) will achieve sustained type-2 orbitalpaths 60 around the axis 28. On the other hand, the ions of mass (M₁)will maintain type-1 orbital paths 62 and remain eccentric to the axis28. Stated differently, the ions of mass (M₁) will not cross the nullcusp 38 and, instead, will be diverted into the radial collector 30.

While the particular Cusp Filter as herein shown and disclosed in detailis fully capable of obtaining the objects and providing the advantagesherein before stated, it is to be understood that it is merelyillustrative of the presently preferred embodiments of the invention andthat no limitations are intended to the details of construction ordesign herein shown other than as described in the appended claims.

What is claimed is:
 1. A cusp filter for altering a multi-species plasmato separate ions of a first mass (M₁) from ions of a second mass (M₂)which comprises: a first magnetic assembly for generating a firstmagnetic field having a magnitude, B, with magnetic field lines orientedin a first direction along an axis; a second magnetic assembly forgenerating a second magnetic field having a magnitude substantiallyequal to B, and having magnetic field lines oriented in a seconddirection along said axis, said second magnetic field opposing saidfirst magnetic field to establish a null cusp oriented substantiallyperpendicular to said axis between said first and second magneticfields; and an injector for directing the ions (M₁) and the ions (M₂)with respective energies along said axis toward said null cusp to divertthe ions (M₁) away from the axis and prevent them from crossing saidnull cusp, while allowing the ions (M₂) to cross the null cusp andproceed along said axis through said filter.
 2. A cusp filter as recitedin claim 1 wherein the magnitude of B is selected to identify a cut-offmass, M_(c), such that M₁<M_(c)<M₂ with M_(c)=e²B²r²/2W where “e” is theion charge, “r” is the radial distance of an ion from the axis in thefirst magnetic field, and W is the kinetic energy of the ion.
 3. A cuspfilter as recited in claim 2 further comprising: a container fordefining a chamber with said first magnetic assembly and said secondmagnetic assembly being, respectively, a plurality of magnetic coilsmounted on said container for generating said first magnetic field andsaid second magnetic field in said chamber; and a vacuum pump connectedto said container for maintaining the multi-species plasma below acollisional density in said chamber, the collisional density beingdefined as a density wherein an ion can cross the null cusp beforesuffering a collision with another ion and the density satisfies acondition wherein the ratio of a collision frequency of an ion to itscyclotron frequency is less than the ratio of r to the axial distance ofthe ion to the null cusp.
 4. A cusp filter as recited in claim 3 whereinsaid null cusp defines a plane and said filter further comprises: aradial collector mounted on said container and oriented substantially insaid plane of said null cusp for collecting ions (M₁) as they arediverted away from said axis; and an axial collector mounted on saidcontainer and positioned substantially on said axis for collecting theions (M₂) as they proceed along said axis through said filter.
 5. A cuspfilter as recited in claim 3 further comprising a means for biasing saidfirst magnetic field to produce a radial electric field for uniformlyincreasing the energies of the ions (M₁) and the ions (M₂) to reduce thesensitivity of M_(c) to r.
 6. A cusp filter as recited in claim 1wherein said energy of the ions (M₁) is substantially equal to saidenergy of the ions (M₂).
 7. A cusp filter as recited in claim 1 whereinsaid injector directs the ions (M₁) and the ions (M₂) toward said nullcusp at a substantially common axial velocity.
 8. A cusp filter asrecited in claim 1 wherein the ions (M₁) have a first resonant frequency(f₁) and the ions (M₂) have a second resonant frequency (f₂) and whereinthe cusp filter further comprises a cyclotron harmonics accelerator,said cyclotron harmonics accelerator being operable to resonate with theions (M₂) to substantially raise the energy of the ions (M₂) above theenergy of the ions (M₁).
 9. A cusp filter as recited in claim 9 whereinsaid cyclotron harmonics accelerator is operated at a frequency of 2f₂.10. A cusp filter as recited in claim 9 wherein said cyclotron harmonicsaccelerator is a quadrant antenna.
 11. A cusp filter as recited in claim1 wherein there are residual ions (M₁) in said second magnetic field andsaid cusp filter further comprises a third magnetic assembly forgenerating a third magnetic field having a magnitude substantially equalto B, and having magnetic field lines oriented in said first directionalong said axis, said third magnetic field opposing said second magneticfield to establish an additional null cusp oriented substantiallyperpendicular to said axis between said second magnetic field and saidthird magnetic field to divert ions (M₁) away from the axis to enhancethe separation of ions (M₁) from ions (M₂) in the plasma.
 12. A cuspfilter for altering a multi-species plasma to separate ions of a firstmass (M₁) from ions of a second mass (M₂) which comprises: a means forgenerating a null cusp defined by a zero magnetic flux function (ψ=0),said null cusp being positioned between a first region having a positivemagnetic flux function (+ψ) and a second region opposed to said firstregion and having a negative magnetic flux function (−ψ), said null cuspbeing substantially planar and oriented substantially perpendicular toan axis; and an injector for directing the ions (M₁) and the ions (M₂)along said axis toward said null cusp with respective energies to divertthe ions (M₁) away from the axis and prevent them from crossing saidnull cusp, while allowing the ions (M₂) to cross the null cusp andproceed along said axis through said filter.
 13. A cusp filter asrecited in claim 12 wherein said generating means comprises: a firstmagnetic assembly for generating a first magnetic field having amagnitude, B, and having said positive magnetic flux function (+ψ) toorient magnetic field lines of said first magnetic field in a firstdirection along an axis; and a second magnetic assembly for generating asecond magnetic field having a magnitude substantially equal to B, andhaving said negative magnetic flux function (−ψ) to orient magneticfield lines of said second magnetic field in a second direction alongsaid axis, said second magnetic field opposing said first magnetic fieldto establish said null cusp between said first and second magneticfields.
 14. A cusp filter as recited in claim 13 wherein the magnitudeof B is selected to identify a cut-off mass, M_(c), such thatM₁<M_(c)<M₂ with M_(c)=e²B²r²/2W where “e” is the ion charge, “r” is theradial distance of an ion from the axis in the first magnetic field, andW is the kinetic energy of the ion.
 15. A cusp filter as recited inclaim 12 wherein said energy of the ions (M₁) is substantially equal tosaid energy of the ions (M₂).
 16. A cusp filter as recited in claim 12wherein the ions (M₁) and the ions (M₂) have a substantially commonaxial velocity.
 17. A cusp filter as recited in claim 12 wherein theions (M₁) have a first resonant frequency (f₁) and the ions (M₂) have asecond resonant frequency (f₂) and wherein the cusp filter furthercomprises a quadrant antenna operable at a frequency of 2f₂ to resonatewith the ions (M₂) to substantially raise the energy of the ions (M₂)above the energy of the ions (M₁).
 18. A method for altering amulti-species plasma to separate ions of a first mass (M₁) from ions ofa second mass (M₂) which comprises the steps of: generating a null cusphaving a zero magnetic flux function (ψ=0), said null cusp beingpositioned between a first region having a positive magnetic fluxfunction (+ψ) and a second region opposed to said first region andhaving a negative magnetic flux function (−ψ), said null cusp beingsubstantially planar and oriented substantially perpendicular to anaxis; and directing the ions (M₁) and the ions (M₂) along said axistoward said null cusp with respective first and second energies todivert the ions (M₁) away from the axis and prevent them from crossingsaid null cusp, while allowing the ions (M₂) to cross the null cusp andproceed along said axis through a cusp filter.
 19. A method as recitedin claim 18 wherein said generating step comprises the steps of:generating a first magnetic field with said positive magnetic fluxfunction (+ψ) and having a magnitude, B, with magnetic field linesoriented in a first direction along an axis; and generating a secondmagnetic field with said negative magnetic flux function (−ψ) and havinga magnitude substantially equal to B, with magnetic field lines orientedin a second direction along said axis, said second magnetic fieldopposing said first magnetic field to establish said null cusp betweensaid first and second magnetic fields.
 20. A method as recited in claim19 further comprising the step of selecting the magnitude of B toidentify a cut-off mass, M_(c), such that M₁<M_(c)<M₂ withM_(c)=e²B²r²/2W where “e” is the ion charge, “r” is the radial distanceof an ion from the axis in the first magnetic field, and W is thekinetic energy of the ion.
 21. A method as recited in claim 20 furthercomprising the step of biasing said first magnetic field to produce aradial electric field for uniformly increasing the energies of the ions(M₁) and the ions (M₂) to reduce the sensitivity of M_(c) to r.
 22. Amethod as recited in claim 18 wherein the ions (M₁) have an energysubstantially equal to the energy of the ions (M₂).
 23. A method asrecited in claim 18 wherein the ions (M₁) and the ions (M₂) have asubstantially common axial velocity.
 24. A method as recited in claim 18wherein the ions (M₁) have a first resonant frequency (f₁) and the ions(M₂) have a second resonant frequency (f₂) and wherein said methodfurther comprises the step of resonating the ions (M₂) with a frequencyof 2f₂ to substantially raise said second energy above said firstenergy.
 25. A method as recited in claim 18 further comprising the stepsof: positioning a radial collector substantially in said planar of saidnull cusp for collecting the ions (M₁) as they are diverted away fromsaid axis; and positioning an axial collector substantially on said axisfor collecting the ions (M₂) as they proceed along said axis throughsaid filter.